In conventional color electrophotogrphy a series of electrostatic images are created on an image member. They are toned with different colored toners and then transferred in registration to a receiving surface to create a multicolor toner image. Typically, the receiving surface is a receiving sheet of paper or similar material which has been secured around the periphery of a transfer drum. The transfer drum is rotated in contact or near contact with the image member to repeatedly bring the receiving sheet into transfer relation with the consecutive images to overlay them in registration.
It is also known to transfer a series of color toner images in registration directly to the periphery of the transfer drum to create the multicolor image on that surface from which it is transferred in a single step to a receiving sheet.
As color toners get finer, for example, less than 10 microns, especially less than 3.5 microns, higher resolutions become possible, even approaching that of conventional silver halide photography. A limiting aspect of the process is the image-to-image registration provided by the transfer process.
Some transfer processes bring the receiving surface into light contact or just out of contact with the image member and transfer is accomplished by an electrostatic field. Using encoders and separate motors on both the image member and the transfer drum, accurate registration from image-to-image is possible with such systems. See, for example, U.S. Pat. Nos. 4,796,054 and 4,872,037.
However, fine toners do not generally transfer well electrostatically. Transfer systems using substantial amounts of pressure, sometimes in the presence of sufficient heat to soften or sinter the toner have been more successful in transferring fine toners. Further, for highest quality work, a receiving sheet with a heat softenable thermoplastic outer layer can be used to receive the toner in the presence of sufficient heat to soften the outer layer and soften or sinter the toner.
These transfer processes require more pressure than is common with ordinary electrostatic transfer. Unfortunately, if both the transfer drum and the image member are independently driven at more than light pressures, excessive wear will destroy a normal image member, for example, a photoconductive member, quite rapidly.
If the transfer drum is driven by the image member, such wear does not occur but registration is difficult to maintain. Even with precisely machined devices there is drift from image-to-image and also over time. Of most significance are slight misregistrations which would not be objectionable in an ordinary color copier, but would be objectionable in a quality photographic print. Also of significance, as the registration drifts, the image may be transferred out of registration with the means for holding the receiving sheet to the periphery of the transfer drum, for example, vacuum holes or gripping fingers.